Diffractive optical element including a diffractive grating pattern

ABSTRACT

Disclosed is a diffractive optical element which includes a diffractive grating pattern on a base plate. The diffractive grating pattern includes a plurality of phase gratings arranged in parallel lines extending along a predetermined direction to cause diffraction of an incident beam. Each of the plurality of phase gratings has an asymmetrical phase pattern. There is a phase gap, ΔP, representing a phase difference (in radians) between an end point and a beginning point of each phase pattern. The phase gap, ΔP, is substantially equal for each of the plurality of phase gratings and satisfies the relationship. 
     
       
           0.7 π&lt;|ΔP|&lt; 1.2π.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No. 09/400,478, filed Sep. 21, 1999, now. U.S. Pat. No. 6,212,016 issued on Apr. 3, 2001, which is a division of U.S. patent application Ser. No. 08/890,429, filed Jul. 9, 1997, now U.S. Pat. No. 6,021,000, issued on Feb. 1, 2000, the contents of which are incorporated herein in their entireties.

BACKGROUND OF THE INVENTION

The present invention relates to a beam splitting optical element which divides an incident beam into a plurality of number of emitted beams, and more particularly, to a beam splitting optical element using diffractive gratings.

Conventionally, beam splitters using diffractive gratings have been known. In such beam splitters, linear grooves or raised portions (i.e., gratings) are formed on, for example, a glass substrate. The arrangement of the gratings determines the pattern of emitted diffracted beams. Typically, the emitted beams (±1st order beams, ±2nd order beams, . . . ) are arranged symmetrically around a central beam (i.e., a zero order diffracted beam) and, as a result, there are an odd number of diffracted beams emitted.

A diffraction efficiency of the conventional diffractive gratings as described above is generally in a range of 70%-85%. There is a need for a beam splitter employing diffractive gratings which has a relatively high diffraction efficiency.

Further, in the field of digital opto-electronics, it is particularly useful if a diffractive optical element has an even number of emitted beams having relatively similar intensities. For example, in an optical recording device accessed by a computer or an optical computer, eight bits (a byte) is a unit when data is processed. If a beam is divided into an even and desired number of beams by the beam splitter, it is advantageous since the emitted beams are used for processing the data efficiently.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide an improved beam splitting optical element which divides an incident beam into an even number of beams and has a higher diffraction efficiency than a conventional element.

For the above object, according to one aspect of the invention, there is provided a diffractive optical element, comprising a cylindrical surface provided with a diffractive grating pattern. The diffractive grating pattern includes a plurality of phase gratings arranged in parallel lines extending along a circumference of the cylindrical surface to cause diffraction of an incident beam, where a beam incident on the diffractive grating pattern is emitted as divided into a plurality of diffracted beams. Since the grating pattern is formed on a cylindrical surface, and the garatings extend along the circumference of the cylindrical surface, a mold to be used for molding the grating pattern can be made easily with use, for example, a lathe.

Preferably, a surface of the optical element from which the diffracted beams are emitted is also cylindrical having a curvature that is substantially the same as a curvature of the cylindrical surface, so that the phase diffracting element has a meniscus shape and has substantially no magnifying power in total.

Optionally, the plurality of phase gratings are of equal width in a direction of the generatrix of the cylinder and each of the plurality of phase gratings has a continuous, nonlinear surface to cause phase differences in a wave front of the incident beam. The mold for such a grating can be made relatively easily when the lathe is used.

Further optionally, each of the plurality of phase gratings has an asymmetrical phase pattern, and a phase gap ΔP, representing a phase difference between an end point of each of the plurality of phase patterns and a beginning point of each of the plurality of phase patterns, in radians. The phase gap, ΔP, is substantially equal for each of the plurality of phase gratings and satisfies:

0.7π<|ΔP|<1.2π.

With this structure, the emitted beams (i.e., the diffracted beams) distribute asymmetrically with respect to the zero order diffracted beam, and accordingly, it is possible that the number of diffracted beams can be adjusted to an even number.

Further optionally, the plurality of phase gratings are adjusted so that each of the divided diffracted beams have substantially the same intensity and no divided beam is omitted other than the intended number of beams. As a result, an even number of diffracted beams have substantially the same intensity may be emitted from the diffractive optical element.

According to another aspect of the invention, there is provided a diffractive optical element, comprising a base plate provided with a diffractive grating pattern. The diffractive grating pattern includes a plurality of phase gratings arranged in parallel lines extending along a predetermined direction of the base plate to cause diffraction of an incident beam. A beam incident on the diffractive grating pattern is emitted as a plurality of diffracted beams, wherein each of the plurality of phase gratings has an asymmetrical phase pattern in a direction where the plurality of phase gratings are arranged A phase gap ΔP, representing a phase difference between an end point of each of the plurality of phase patterns and a beginning point of each of the plurality of phase patterns, in radians, is substantially equal for each of the plurality of phase gratings and satisfies:

0.7π<|ΔP|<1.2π.

With this optical element, a desired even number of diffracted beams, which are asymmetrically distributed with respect to the zero order beam, are obtained.

It should be noted that the diffracted beams substantially consist of a desired number of beams.

Various examples are indicated as embodiments. In each embodiment, a predetermined error in the phase pattern is permissible.

Specifically, the predetermined permissible error in the phase difference may be less than 2%.

According to a further aspect of the invention, there is provided a diffractive optical element, comprising: a base plate having a cylindrical surface; and a diffractive grating pattern engraved on the cylindrical surface in a direction perpendicular to a generatrix of the cylindrical surface so that diffracted beams distribute in a dimension along the generatrix.

Since the diffractive grating pattern is formed on the cylindrical surface, a mold to be used for molding the optical element can be produced relatively easily.

Optionally, the grating pattern includes a plurality of phase gratings. Due to the shape of the optical element, and therefore the shape of the mold for the optical element, a complicated pattern can be employed. Accordingly, the grating can be a phase grating. When employing the phase grating, diffraction efficiency is improved.

Optionally or alternatively, each of the phase gratings has an asymmetrical phase pattern. As a result, the diffracted beams distribute asymmetrically with respect to a zero order diffracted beam.

Accordingly, by selecting an appropriate phase pattern of the phase gratings, an even number of diffracted beams can be emitted.

According to a further aspect of the invention, there is provided a method for producing a diffracting optical element, comprising: making a mold by (1) rotating a cylindrical metal mold about a rotation axis, and (2) moving a cutting tool to a predetermined radial distance from the rotation axis and moving the tool along the rotation axis; and applying an injection mold process with use of a master made by the steps in making the mold to make the diffracting optical element.

With this method, a complicated phase pattern can be formed on the mold.

According to a further aspect of the invention, there is provided a method for producing a mold to be used for making a diffracting optical element having a cylindrical surface with an injection mold process, comprising (1) rotating a cylindrical metal mold about a rotation axis, and (2) moving a cutting tool to a predetermined radial distance from the rotation axis and moving the tool along the rotation axis. Also with this method, a complicated pattern extending along a circumference of the mold can be formed on the circumferential surface of the mold.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic enlarged perspective view of gratings formed on a beam splitter according to an embodiment of the invention;

FIG. 2 is a perspective view of the beam splitter according to an embodiment of the invention;

FIG. 3 is a perspective view illustrating a process for making a mold for the beam splitter of FIG. 2;

FIGS. 4 through 15 are graphs illustrating exemplary phase patterns for the beam splitter of FIG. 2;

FIGS. 16 through 27 are graphs showing a distribution of intensities of the diffracted beams corresponding to the exemplary phase patterns of FIGS. 4 through 15; and

FIG. 28 is a perspective view of the beam splitter according to an alternative embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 is a perspective view of a beam splitter 10 according to an embodiment of the invention. The beam splitter 10 includes a base 11 on which a grating pattern 12 is formed. As shown in FIG. 2, the base 11 has a concave surface 11 a and a convex surface 11 b and is represented as a portion of a wall of a cylinder. In FIG. 2, dotted lines show an imaginary cylinder with ends C1 and C2 having equal diameters representing a diameter of the concave surface 11 a. The grating pattern 12 is formed on the concave surface 11 a.

FIG. 1 is a schematic enlarged view of the grating pattern 12. It should be noted that in FIG. 1, the grating pattern 12 is shown as formed on a flat surface, and description with reference to FIG. 1 is made as if the grating pattern 12 is formed on the flat surface. As described above, however, the grating pattern 12 is actually formed on the concave surface 11 a.

The grating pattern 12 is formed having a plurality of identically formed phase gratings P extending linearly. Each of the phase gratings P have a predetermined length L along a y-axis direction and linearly extend in an x-axis direction. In particular, the phase gratings shown in FIG. 1 correspond to a particular numerical example (example 3) described in more detail below. Note that the y-axis direction is a direction parallel to a generatrix of the base 11 (i.e., the imaginary cylinder defined by the end circles C1 and C2) and the x-axis direction represents the curve of the concave surface 11 a.

The grating pattern 12 and the base 11 are made of, for example, glass or a transparent resinous material. The grating pattern 12 is formed so that it divides an incident beam into a plurality of diffracted emitted beams. It should be noted that the grating pattern can be formed on a flat surface. Practically, however, due to difficulty in producing a complicated grating pattern as described below, it is preferable to employ a cylindrical surface as a surface on which the grating pattern 12 is formed. Further, if the cylindrical surface is employed, it may be advantageous to use resinous material for the optical element, and the optical element may be formed with a molding process.

As shown in FIG. 1, a cross section along a Y-Z plane of each phase grating P has a shape which causes non-linear beams to pass therethrough along directions represented by the arrows D, E in FIG. 2. In other words, lines representative of the phase difference caused by the beam splitter 10 is similar to the cross-sectional shape of the phase gratings P. If the phase difference caused by the grating is represented by δ with respect to the lowermost portion of the surface of the phase grating P, the surface of the phase grating P is defined by a nonlinear phase difference δ which varies along the y-axis direction. In other words, the phase difference δ is defined as a difference between a point on a phase grating P and a predetermined reference point on the phase grating P. In the embodiments, the predetermined reference point is determined and meets with an adjacent phase grating P with a phase gap ΔP. In particular, the phase gap ΔP should be constant for all phase gratings P and should satisfy the condition:

0.7π<|ΔP|<1.2π.

According to the beam splitter 10 of the embodiment, the cross-section along the y-axis phase gratings P have an asymmetrical shape (i.e., an asymmetrical phase distribution), and the diffracted beams are also asymmetrical with respect to a zero order diffracted beam and the incident beam is divided into an even number of emitted beams.

A method of forming the beam splitter 10 is now described. Since the cross-sectional shape of the phase patterns P is nonlinear and complicated, it is difficult to form a mold for the grating pattern 12 using an etching process. Accordingly, the diffractive optical element 10 is molded using a metal mold. However, if the surface on which a master pattern is formed is a flat surface, a cutting tool for forming the pattern is to be moved three dimensionally, i.e., in the x, y and z axis directions relative to the surface. Considering the size of the gratings, it may be very difficult to control the movement of the cutting tool to form the pattern precisely.

FIG. 3 is a perspective view illustrating a process for making a mold 30 for the beam splitter 10. Since each of the gratings has a surface which cannot be made with use of an etching or the like, the mold 30 should be used in order to make the phase gratings. The mold 30 is a cylindrical member as shown in FIG. 2, and the pattern 31, representing the grating pattern 12, is formed on the circumferential surface of the mold 30 using a cutting tool 100. The mold 30 is then used to form the beam splitter 10. When the beam splitter 10 is made, for example, a well-known injection mold method is applied using the mold 30 as a master. It should be noted that the beam splitter 10 is formed of optical plastic such as PMMA (Plymethyl methacrylate).

In this embodiment, as shown in FIG. 2, since the phase difference δ of each phase pattern P is constant along the x-axis direction, the positional relation between the mold 30 and the cutting tool 100 need only be adjusted in two dimensions (i.e., the y-axis direction and the z-axis direction). Thus, the phase patterns P can be engraved accurately in a short time and at a low cost.

As shown in FIG. 3, the cutting tool 100 includes a lathe 20, a support 21 rotated by the lathe 20 and movable along a rotation axis thereof (i.e., movable in a y-axis direction), a sliding table 22 arranged to move perpendicular to the rotation axis of the support 21 (i.e., in a z-axis direction), and a tool 23 fixedly provided on the sliding table 22.

The metal mold 30 is fixed coaxially with the support 21 and is rotated in a direction Rx (corresponding to the x-axis direction of FIG. 1). Then, by appropriate movement of the support 21 along the y-axis direction and of the sliding table 22 along the z-axis direction with the metal mold 30 rotated, the mold 30 for the phase gratings P is formed.

The mold formed 30 is then used to form the beam splitter 10.

Twelve particular numerical examples of the phase grating P are now described with reference to FIGS. 4 through 27.

In example 1, the phase grating P is formed such that an incident beam is divided into 4 emitted beams by the beam splitter 10, in examples 2-5, the phase grating P is formed such that an incident beam is divided into 8 emitted beams, and in examples 6-12, the phase grating P is formed such that an incident beam is divided into 16 emitted beams by the beam splitter 10.

In these examples, the phase grating P is designed such that: (1) intensities of each emitted beam are substantially the same, and (2) only the intended number of emitted beams are emitted.

In the following description, the width L (i.e., the length of in the Y-axis direction) of each phase grating P is divided into 64 co-ordinates (designated 0-63). A reference point is designated as a point at which the phase pattern P is lowest in relation to the concave surface 11 a. Further, the phase difference δ for each co-ordinate is given in radians. Accordingly, the phase difference δ is more than 0. However, a height H along the z-axis direction (i.e., an actual height of the phase grating) in micrometers (μm) may be calculated, for a predetermined incident beam, using the formula:

H=δ×λ/(2π(n−1)),

where n is a refractive index of the material of the beam splitter 10 and λ is a wavelength of the incident beam. It is assumed that the beam splitter 10 is located within air whose refractive index is regarded as 1.

EXAMPLE 1

Table 1 shows data for a pattern of the phase grating P according to example 1. The data is shown graphically in FIG. 4 where a vertical axis is the phase difference δ and a horizontal axis is the coordinate along the y-axis direction. In example 1, the phase gap ΔP is 1.00π. Note that, the phase gap ΔP is defined as a difference between the phases at the coordinate 0 and the coordinate 63.

TABLE 1 (ΔP = 1.00π) Coord. δ Coord. δ Coord. δ  0 0.00000 22 2.28947 44 2.89397  1 0.04000 23 2.51947 45 2.79397  2 0.10042 24 2.69518 46 2.74279  3 0.16042 25 2.86518 47 2.69279  4 0.21520 26 3.02656 48 2.67291  5 0.25520 27 3.14656 49 2.65291  6 0.32315 28 3.25191 50 2.66056  7 0.38315 29 3.35191 51 2.67056  8 0.44920 30 3.41679 52 2.67518  9 0.51920 31 3.48679 53 2.69518 10 0.59142 32 3.52997 54 2.72447 11 0.66142 33 3.55997 55 2.75447 12 0.75100 34 3.59951 56 2.79033 13 0.84100 35 3.59951 57 2.82033 14 0.94020 36 3.58770 58 2.86386 15 1.05020 37 3.55770 59 2.89386 16 1.17834 38 3.50415 60 2.94434 17 1.30834 39 3.44415 61 2.98434 18 1.46586 40 3.33870 62 3.03420 19 1.64586 41 3.21870 63 3.13420 20 1.84933 42 3.10951 21 2.06933 43 2.96951

EXAMPLE 2

Table 2 shows data for a pattern of the phase grating P according to example 2. The data is shown graphically in FIG. 5. In example 2, the phase gap ΔP is 0.75π.

TABLE 2 (ΔP = 0.75) Coord. δ Coord. δ Coord. δ  0 2.34750 22 2.33100 44 6.16100  1 2.11350 23 2.54700 45 6.29850  2 2.10350 24 2.76700 46 6.38850  3 2.11700 25 2.98600 47 6.46600  4 2.13200 26 3.23600 48 6.51600  5 2.14700 27 3.57850 49 6.53700  6 2.14700 28 4.10850 50 6.48700  7 2.10350 29 4.68100 51 6.38100  8 1.96350 30 5.10100 52 6.17100  9 1.32750 31 5.39450 53 5.85000 10 0.36750 32 5.57450 54 5.47000 11 0.07000 33 5.70650 55 5.22600 12 0.00000 34 5.79650 56 5.06600 13 0.02550 35 5.84250 57 4.99800 14 0.12550 36 5.88250 58 4.94800 15 0.23800 37 5.89250 59 4.93550 16 0.40800 38 5.89250 60 4.92550 17 0.65600 39 5.89650 61 4.91000 18 0.96600 40 5.90650 62 4.89000 19 1.34000 41 5.91450 63 4.70250 20 1.72000 42 5.96450 21 2.05100 43 6.06100

EXAMPLE 3

Table 3 shows data for a pattern of the phase grating P according to example 3. The data is shown graphically in FIG. 6. In example 3, the phase gap ΔP is 0.99π.

TABLE 3 (ΔP = 0.99π) Coord. δ Coord. δ Coord. δ  0 0.00000 22 5.53800 44 5.95500  1 0.04000 23 5.69800 45 5.74500  2 0.08700 24 5.81200 46 5.57400  3 0.16700 25 5.87200 47 5.43400  4 0.24600 26 5.88400 48 5.33000  5 0.35600 27 5.85400 49 5.23000  6 0.45900 28 5.78500 50 5.11400  7 0.63900 29 5.70500 51 4.94400  8 0.88800 30 5.63700 52 4.63400  9 1.31800 31 5.63700 53 4.11400 10 1.99700 32 5.67800 54 3.52200 11 2.65700 33 5.79800 55 3.20200 12 3.03300 34 5.95300 56 3.03800 13 3.30300 35 6.12300 57 2.95800 14 3.52200 36 6.28800 58 2.94800 15 3.71200 37 6.41800 59 2.95800 16 3.91200 38 6.51300 60 2.96900 17 4.15200 39 6.55300 61 2.98900 18 4.41000 40 6.53000 62 3.03300 19 4.73000 41 6.47000 63 3.09800 20 5.03600 42 6.35300 21 5.32600 43 6.16300

EXAMPLE 4

Table 4 shows data for a pattern of the phase grating P according to example 4. The data is shown graphically in FIG. 7. In example 4, the phase gap ΔP is 0.99π.

TABLE 4 (ΔP = 0.99π) Coord. δ Coord. δ Coord. δ  0 1.57900 22 2.15900 44 2.38400  1 1.63900 23 2.39900 45 2.11400  2 1.66700 24 2.60700 46 1.93200  3 1.69700 25 2.81700 47 1.81200  4 1.73900 26 3.04600 48 1.74600  5 1.74400 27 3.25600 49 1.75600  6 1.73000 28 3.49200 50 1.82500  7 1.70000 29 3.71200 51 1.96500  8 1.55400 30 3.90500 52 2.26500  9 1.25400 31 4.02500 53 2.82500 10 0.71100 32 4.06700 54 3.47800 11 0.25100 33 4.04700 55 3.87800 12 0.05000 34 3.97200 56 4.09500 13 0.00000 35 3.84200 57 4.23500 14 0.04100 36 3.70100 58 4.34700 15 0.12100 37 3.57100 59 4.43790 16 0.20200 38 3.44500 60 4.50400 17 0.50200 39 3.33500 61 4.55400 18 0.80200 40 3.22000 62 4.62600 19 1.15200 41 3.09000 63 4.68600 20 1.52900 42 2.90600 21 1.87900 43 2.65600

EXAMPLE 5

Table 5 shows data for a pattern of the phase grating P according to example 5. The data is shown graphically in FIG. 8. In example 5, the phase gap ΔP is 1.00π.

TABLE 5 (ΔP = 1.00π) Coord. δ Coord. δ Coord. δ  0 0.00000 22 5.13656 44 5.80812  1 0.03494 23 5.21307 45 5.60323  2 0.08372 24 5.20778 46 5.37918  3 0.17593 25 5.13788 47 5.12460  4 0.29127 26 4.97072 48 4.88807  5 0.41957 27 4.73558 49 4.68950  6 0.57022 28 4.48842 50 4.48952  7 0.77908 29 4.32697 51 4.29759  8 1.04067 30 4.24668 52 4.05168  9 1.33889 31 4.24576 53 3.80124 10 1.70048 32 4.27485 54 3.56782 11 2.04910 33 4.39394 55 3.34777 12 2.38318 34 4.58241 56 3.19436 13 2.68329 35 4.84203 57 3.08368 14 2.97144 36 5.19737 58 3.01120 15 3.26819 37 5.51068 59 2.97107 16 3.58289 38 5.77602 60 2.96391 17 3.93565 39 5.94409 61 2.98987 18 4.28787 40 6.03755 62 2.99927 19 4.60094 41 6.07922 63 3.13750 20 4.85049 42 6.05674 21 5.03591 43 5.96950

EXAMPLE 6

Table 6 shows data for a pattern of the phase grating P according to example 6. The data is shown graphically in FIG. 9. In example 6, the phase gap ΔP is 1.01π.

TABLE 6 (ΔP = 1.01π) Coord. δ Coord. δ Coord. δ  0 0.00000 22 9.01142 44 7.39884  1 0.11909 23 9.28050 45 7.47793  2 0.38167 24 9.40910 46 7.70302  3 0.86576 25 9.51318 47 8.32711  4 1.60085 26 9.57577 48 8.88019  5 2.24994 27 9.42985 49 8.91927  6 2.70552 28 8.82495 50 8.63937  7 3.15461 29 8.55903 51 8.24345  8 3.79220 30 8.57712 52 7.86954  9 4.65129 31 8.64120 53 7.49862 10 5.33787 32 8.69029 54 6.90022 11 5.83696 33 8.68438 55 6.09930 12 6.33905 34 8.73447 56 5.52989 13 6.84314 35 9.13856 57 5.20897 14 7.22122 36 9.91164 58 4.85157 15 7.25031 37 10.12573 59 4.33065 16 6.79540 38 10.16132 60 3.66374 17 6.30948 39 10.14541 61 3.27782 18 6.15257 40 10.08499 62 3.13342 19 6.17165 41 9.91408 63 3.17750 20 6.27975 42 8.31967 21 7.34883 43 7.42876

EXAMPLE 7

Table 7 shows data for a pattern of the phase grating P according to example 7. The data is shown graphically in FIG. 10. In example 7, the phase gap ΔP is 0.98π.

TABLE 7 (ΔP = 0.98π) Coord. δ Coord. δ Coord. δ  0 0.00000 22 9.18279 44 8.33523  1 0.10112 23 9.81372 45 7.85733  2 0.21482 24 10.31935 46 7.25375  3 0.46317 25 10.74935 47 6.51311  4 0.87331 26 11.07071 48 5.96011  5 1.39192 27 11.23482 49 5.55847  6 1.73829 28 11.08046 50 5.07889  7 1.78828 29 10.43581 51 4.47400  8 1.74891 30 9.96051 52 4.01530  9 1.68984 31 9.86263 53 3.87218 10 1.76142 32 9.91072 54 3.90177 11 2.02072 33 10.11877 55 4.04902 12 2.55159 34 10.68425 56 4.20056 13 3.28166 35 11.42907 57 4.21774 14 3.83542 36 11.68461 58 4.00155 15 4.34223 37 11.60267 59 3.56311 16 4.99540 38 11.38748 60 3.24915 17 5.83822 39 11.05666 61 3.11798 18 6.55497 40 10.65121 62 3.08845 19 7.10304 41 10.11445 63 3.09300 20 7.68791 42 9.43005 21 8.41221 43 8.81092

EXAMPLE 8

Table 8 shows data for a pattern of the phase grating P according to example 8. The data is shown graphically in FIG. 11. In example 8, the phase gap ΔP is 1.14π.

TABLE 8 (ΔP = 1.14) Coord. δ Coord. δ Coord. δ  0 0.00000 22 8.14593 44 6.38976  1 0.35675 23 8.06924 45 6.15963  2 0.59145 24 7.78458 46 5.82247  3 1.02421 25 7.15265 47 5.41843  4 1.76643 26 6.44611 48 4.98783  5 2.45950 27 5.97778 49 4.50606  6 2.97905 28 5.63530 50 4.05856  7 3.46447 29 5.27806 51 3.78350  8 4.30512 30 4.81668 52 3.74228  9 5.26163 31 4.14179 53 4.15449 10 5.78634 32 3.46774 54 5.00983 11 6.07644 33 3.08316 55 5.34813 12 6.23928 34 2.88663 56 5.39878 13 6.33414 35 2.84806 57 5.22764 14 6.27698 36 3.03808 58 4.60923 15 6.11553 37 3.51615 59 3.77745 16 5 97524 38 4.17024 60 3.46904 17 5.92432 39 4.54980 61 3.39766 18 6.05341 40 4.92638 62 3.41174 19 6.40250 41 5.62633 63 3.59185 20 7.41097 42 6.27292 21 8.03059 43 6.49224

EXAMPLE 9

Table 9 shows data for a pattern of the phase grating P according to example 9. The data is shown graphically in FIG. 12. In example 9, the phase gap ΔP is 0.86π.

TABLE 9 (ΔP = 0.86π) Coord. δ Coord. δ Coord. δ  0 0.00000 22 10.41512 44 11.31028  1 0.02139 23 10.45448 45 11.04914  2 0.24455 24 10.66319 46 10.70588  3 0.63197 25 11.05519 47 10.49409  4 1.25970 26 11.52474 48 10.31520  5 1.93395 27 11.90449 49 10.09816  6 2.45426 28 12.17922 50 9.41989  7 2.94875 29 12.38914 51 8.18725  8 3.67947 30 12.57125 52 7.75399  9 4.63407 31 12.71915 53 7.51219 10 5.28644 32 12.78723 54 7.14200 11 5.75541 33 12.73151 55 6.22557 12 6.29335 34 12.67358 56 5.34703 13 7.22466 35 12.59483 57 4.96071 14 8.10410 36 12.48228 58 4.61458 15 8.59831 37 12.25170 59 4.07851 16 8.96738 38 11.83332 60 3.42295 17 9.36434 39 11.38850 61 3.06471 18 9.85078 40 11.15996 62 2.90472 19 10.26310 41 11.13878 63 2.69100 20 10.46203 42 11.22934 21 10.45751 43 11.33304

EXAMPLE 10

Table 10 shows data for a pattern of the phase grating P according to example 10. The data is shown graphically in FIG. 13. In example 10, the phase gap ΔP is 1.07π.

TABLE 10 (ΔP = 1.07π) Coord. δ Coord. δ Coord. δ  0 1.88359 22 6.11453 44 0.00000  1 2.20660 23 5.47716 45 0.11027  2 2.55505 24 4.92379 46 0.55334  3 2.96194 25 4.32768 47 1.03784  4 3.50884 26 3.82595 48 1.31994  5 4.12613 27 3.35523 49 1.57085  6 4.76767 28 2.90596 50 1.87626  7 5.47822 29 2.32512 51 2.43060  8 6.13673 30 1.74405 52 3.02974  9 6.68710 31 1.41813 53 3.35112 10 7.20112 32 1.33906 54 3.36775 11 7 89495 33 1.50709 55 3.30493 12 9.04040 34 1.88758 56 3.22809 13 9.57330 35 2.18736 57 3.38151 14 9.71868 36 2.27226 58 3.63778 15 9.62977 37 2.04002 59 3.99318 16 9.28356 38 1.53403 60 4.32386 17 8.61084 39 1.01003 61 4.56700 18 8.05756 40 0.69457 62 4.81525 19 7.66091 41 0.47837 63 5.24823 20 7.25906 42 0.31341 21 6.74087 43 0.09924

EXAMPLE 11

Table 11 shows data for a pattern of the phase grating P according to example 11. The data is shown graphically in FIG. 14. In example 11, the phase gap ΔP is 1.04π.

TABLE 11 (ΔP = 1.04π) Coord. δ Coord. δ Coord. δ  0 5.72434 22 3.36629 44 2.81337  1 5.63396 23 3.48561 45 3.34012  2 5.50930 24 3.42313 46 3.80482  3 5.27261 25 3.12300 47 4.35758  4 4.88795 26 2.69584 48 4.96980  5 4.30602 27 2.35180 49 5.69287  6 3.64948 28 1.99120 50 6.46242  7 3.08115 29 1.63443 51 7.23784  8 2.55867 30 1.17193 52 7.81849  9 1.97143 31 0.85687 53 8.30500 10 1.34005 32 0.75565 54 8.79971 11 0.79516 33 0.92786 55 9.44981 12 0.40111 34 1.14320 56 9.93265 13 0.10653 35 1.34150 57 10.15751 14 0.00000 36 1.34215 58 10.06035 15 0.05143 37 1.13101 59 9.80890 16 0.39145 38 0.74260 60 9.48861 17 0.97952 39 0.50082 61 9.27769 18 1.48361 40 0.45241 62 9.11678 19 1.93317 41 0.64103 63 8.98587 20 2.36975 42 1.08511 21 2.92970 43 2.01522

EXAMPLE 12

Table 12 shows data for a pattern of the phase grating P according to example 12. The data is shown graphically in FIG. 15. In example 12, the phase gap ΔP is 0.98π.

TABLE 12 (ΔP = 0.98π) Coord. δ Coord. δ Coord. δ  0 1.54498 22 5.12490 44 4.58932  1 1.53807 23 5.76799 45 4.08540  2 1.47715 24 6.51557 46 3.64850  3 1.24424 25 7.20367 47 3.38558  4 0.73233 26 7.74924 48 3.32567  5 0.22242 27 8.19234 49 3.45876  6 0.00000 28 8.70042 50 3.70385  7 0.04708 29 9.36852 51 3.91694  8 0.33968 30 9.97109 52 3.97602  9 1.19776 31 10.29019 53 3.83511 10 1.79235 32 10.33327 54 3.20469 11 1.98643 33 10.09636 55 2.54279 12 1.97353 34 9.53795 56 2.38836 13 1.83361 35 8.96804 57 2.49146 14 1.66370 36 8.57712 58 2.85804 15 1.65178 37 8.20621 59 3.52314 16 1.84488 38 7.75680 60 4.08221 17 2.25997 39 7.15388 61 4.39631 18 2.82905 40 6.55447 62 4.55539 19 3.44314 41 6.03255 63 4.60899 20 4.01223 42 5.55515 21 4.55932 43 5.07323

Tables 13 and 14 show the output intensity for the emitted beams of the beam splitter 10 in each of the above twelve examples as a relative intensity when the intensity of the incident beam is defined as 1. Further, an effective intensity represents a sum of the intensities of the intended emitted beams as a percentage of the incident beam. As explained above, the intended emitted beams are, for example, in example 1, the four emitted beams of order −1 to +2, or in example 2, the eight emitted beams of order −3 to +4.

FIGS. 16-27 show the data of Tables 13 and 14 graphically, the horizontal axis represents the order of the emitted diffracted beam and the vertical axis represents the intensity of each order where the intensity of the incident beam is defined as 1.

TABLE 13 Order Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 −10     0.00097  0.00396  0.00162  0.00142  0.00142  0.00123 −9    0.00113  0.00850  0.00503  0.00526  0.00136  0.00568 −8    0.00126  0.00762  0.00528  0.00273  0.00244  0.00207 −7    0.00207  0.00171  0.00186  0.00440  0.00068  0.05752 −6    0.00257  0.00093  0.00023  0.00048  0.00513  0.05869 −5    0.00201  0.00156  0.00039  0.00091  0.00011  0.05867 −4    0.00874  0.00584  0.00316  0.00175  0.00007  0.05900 −3    0.01013  0.11779  0.11830  0.11805  0.12016  0.05797 −2    0.00226  0.11917  0.11781  0.11865  0.11955  0.05872 −1    0.22965  0.11753  0.11965  0.11949  0.12001  0.05928 0  0.23019  0.11745  0.11878  0.11879  0.12056  0.05975 1  0.23039  0.11841  0.11876  0.11903  0.12045  0.05970 2  0.22923  0.11717  0.11962  0.11942  0.11997  0.05907 3  0.00231  0.11643  0.11770  0.11859  0.11957  0.05875 4  0.01049  0.11663  0.11824  0.11794  0.12017  0.05798 5  0.00898  0.00126  0.00320  0.00174  0.00004  0.05911 6  0.00213  0.00462  0.00046  0.00092  0.00019  0.05861 7  0.00282  0.00004  0.00022  0.00056  0.00550  0.05882 8  0.00225  0.00227  0.00184  0.00444  0.00073  0.05750 9  0.00137  0.00003  0.00521  0.00279  0.00263  0.00253 10   0.00133  0.00397  0.00499  0.00527  0.00158  0.00658 Effec. 91.95% 94.06% 94.89% 95.00% 96.04% 93.91%

TABLE 14 Order Ex. 7 Ex. 8 Ex. 9 Ex. 10 Ex. 11 Ex. 12 −10     0.00165  0.00019  0.00024  0.00004  0.00052  0.00020 −9    0.00089  0.00053  0.00002  0.00106  0.00117  0.00017 −8    0.00457  0.00028  0.00292  0.00098  0.00028  0.00092 −7    0.06045  0.05997  0.06068  0.06070  0.06086  0.06125 −6    0.06056  0.06019  0.06117  0.06027  0.06086  0.06103 −5    0.06008  0.06018  0.06126  0.06076  0.06076  0.06091 −4    0.06037  0.05995  0.06077  0.06038  0.06097  0.06116 −3    0.06089  0.06015  0.06083  0.06078  0.06078  0.06105 −2    0.06033  0.06017  0.06074  0.06065  0.06085  0.06115 −1    0.06028  0.06058  0.06070  0.06044  0.06101  0.06122 0  0.06020  0.06056  0.06061  0.06092  0.06116  0.06107 1  0.06023  0.06050  0.06051  0.06082  0.06092  0.06105 2  0.06022  0.06060  0.06043  0.06087  0.06099  0.06118 3  0.06041  0.06108  0.06063  0.06111  0.06106  0.06115 4  0.06087  0.06074  0.06066  0.06119  0.06100  0.06095 5  0.06037  0.06055  0.06053  0.06127  0.06107  0.06110 6  0.06008  0.06092  0.06091  0.06057  0.06129  0.06093 7  0.06055  0.06035  0.06083  0.06116  0.06104  0.06102 8  0.06041  0.06129  0.06032  0.06099  0.06120  0.06118 9  0.00463  0.00144  0.00115  0.00113  0.00173  0.00111 10   0.00086  0.00187  0.00028  0.00021  0.00035  0.00026 Effec 96.63% 96.78% 97.16% 97.29% 97.58% 97.74%

As shown in table 13 and 14, the effective intensity of the intended emitted beams is more than 91% in each example and reaches as high as 97.74%.

Tables 15 and 16 show the intensity of the intended emitted beams as a percentage of an ideal value. For instance, in example 1, the ideal value of each emitted beam is 0.25. (4 emitted beams are desired) where the intensity of the incident beam is 1, however, the actual intensity of the emitted beam of −1 order is 0.22965 so that the percentage of the −1 order beam is 92%. Similarly, the ideal values are 0.125 in examples 2-4 (8 emitted beams are desired) and 0.0625 in examples 6-12 (16 emitted beams are desired).

TABLE 15 Order Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 −7   92% −6   94% −5   94% −4   94% −3   94% 95% 94% 96% 93% −2   95% 94% 95% 96% 94% −1   92% 94% 96% 96% 96% 95% 0 92% 94% 95% 95% 96% 96% 1 92% 95% 95% 95% 96% 96% 2 92% 94% 96% 96% 96% 95% 3 93% 94% 95% 96% 94% 4 93% 95% 94% 96% 93% 5 95% 6 94% 7 94% 8 92% Δ  0%  2%  2%  2%  0%  4%

TABLE 16 Order Ex. 7 Ex. 8 Ex. 9 Ex. 10 Ex. 11 Ex. 12 −7   97% 96% 97% 97% 97% 98% −6   97% 96% 98% 96% 97% 98% −5   96% 96% 98% 97% 97% 97% −4   97% 96% 97% 97% 98% 98% −3   97% 96% 97% 97% 97% 98% −2   97% 96% 97% 97% 97% 98% −1   96% 97% 97% 97% 98% 98% 0 96% 97% 97% 97% 98% 98% 1 96% 97% 97% 97% 97% 98% 2 96% 97% 97% 97% 98% 98% 3 97% 98% 97% 98% 98% 98% 4 97% 97% 97% 98% 98% 98% 5 97% 97% 97% 98% 98% 98% 6 96% 97% 97% 97% 98% 97% 7 97% 97% 97% 98% 98% 98% 8 97% 98% 97% 98% 98% 98% Δ  1%  2%  1%  2%  1%  1%

As shown in Tables 15 and 16, the intensities of the intended emitted beams are in a range of 92%-98% in all examples. Further, a difference Δ between maximum and minimum percentage values is at most 4%. Accordingly, in these examples, the energy of the incident beam is effectively equally divided among the intended emitted beams.

The intensities shown in the above Tables and Figures represent ideal values. In considering some errors in the beam diffractive element, the efficiency will be reduced. In particular, if the error in the efficiency is to be under 10%, the permissible error in the pattern of the phase grating P is about 2%. For example, if the refractive index n of the environment is 1.5, the incident beam has a wavelength λ of 488 nm and is to be divided into 8 emitted beams every 0.0125 rad, the pattern has a length L along the y-axis of about 40 μm and a maximum height H along the z-axis of about 1 μm. Here, the height H is defined as a difference between the highest and the lowest points in the phase pattern P. Thus, the permissible error in the height is only 0.02 μm.

However, the permissible error range can be extended. For example, if a difference between the refractive indexes of the phase grating P and the environment is decreased, the size of the phase pattern can be increased. That is, if the difference between refractive indexes is smaller, the height H can be larger. Accordingly, the required processing precision of the beam splitter 10 can be reduced. In a particular case, the concave surface 11 a, including the grating pattern 12, may be covered with a liquid layer having a refractive index that is almost equal to that of the grating pattern 12.

As described above, in each embodiment, the grating pattern is asymmetrical in the direction where the gratings are aligned. Thus the diffracted beams are not symmetrical with respect to zero order beam, and an even number of diffracted beams are generated. Further, the diffraction efficiency is raised by forming the grating pattern to have multi-level phase distribution, and energy of the incident beam is efficiently used.

It should be noted that the phase patterns described above should be optimized so that the diffracted beams consist substantially of a desired number (even number) of beams, and the desired number of beams have substantially the same intensities.

Although the structure and operation of a beam splitter is described herein with respect to the preferred embodiments, many modifications and changes can be made without departing from the spirit and scope of the invention.

For example, as an alternative, the phase patterns P may be formed as substantially indenting into the base 11 (as shown in FIG. 28) rather than as substantially protruding from the base 11 (as shown in FIG. 2). For examples given above, the alternative forms for the phase patterns P can be obtained if the reference point (i.e., the 0 point) remains the same but each of the phase differences δ are defined as negative values. In other words, the cross-section of the phase pattern in the Y-Z plane can be considered to be rotated about the Y-axis by 180 degrees to produce a mirror image of the phase pattern.

The present disclosure relates to subject mater contained in Japanese Patent Application Nos. HEI 08-198271, filed on Jul. 3, 1996, and HEI 08-198272, filed on Jul. 9, 1996, which are expressly incorporated herein by reference in their entirety. 

What is claimed is:
 1. A diffractive optical element, comprising a cylindrical surface provided with a diffractive grating pattern, said diffractive grating pattern including a plurality of phase gratings arranged in parallel lines extending along a circumference of said cylindrical surface to cause diffraction of an incident beam, wherein a beam incident on said diffractive grating pattern is divided into and emitted as a plurality of diffracted beams; and wherein said plurality of phase gratings are of equal width and each of said plurality of phase gratings has an asymmetrical phase pattern and a continuous, nonlinear surface with a starting point and an ending point, such that a position of an ending point of one phase grating coincides with a position of a starting point of an adjacent phase grating in a direction of arrangement of the gratings, said plurality of phase gratings causing phase differences in a wave front of said incident beam, and wherein, a phase gap ΔP, representing a phase difference between an end point of each of said plurality of phase patterns and a beginning point of each of said plurality of phase patterns, in radians, is substantially equal for each of said plurality of phase gratings and satisfies: 0.7π<|ΔP|<1.2π.
 2. The diffractive optical element according to claim 1, wherein said plurality of phase grating are adjusted so that each of said divided diffracted beams has substantially the same intensity and no divided beam is emitted other than said diffracted beams having substantially the same intensity.
 3. A diffractive optical element, comprising a cylindrical surface provided with a diffractive grating pattern, said diffractive grating pattern including a plurality of phase gratings arranged in parallel lines extending along a circumference of said cylindrical surface to cause diffraction of an incident beam, wherein a beam incident on said diffractive grating pattern is divided into and emitted as a plurality of diffracted beams; and wherein said plurality of phase gratings are of equal width and each of said plurality of phase gratings has a continuous nonlinear surface with a starting point and an ending point, such that a position of an ending point of one phase grating coincides with a position of a starting point of an adjacent phase grating, in a direction of arrangement of the gratings, said plurality of phase grating causing phase differences in a wave front of said incident beam, said phase grating being arranged so that a predetermined number of diffracted beams having substantially a same intensity are emitted, and no additional beam having a substantial intensity is emitted.
 4. The diffractive optical element according to claim 3, wherein each of said plurality of phase gratings has an asymmetrical phase pattern, and wherein, a phase gap ΔP, representing a phase difference between an end point of each of said plurality of phase patterns and a beginning point of each of said plurality of phase patterns, in radians, is substantially equal for each of said plurality of phase gratings and satisfies: 0.7π<|ΔP|<1.2π.
 5. A diffractive optical element, comprising a cylindrical surface provided with a diffractive grating pattern, said diffractive grating pattern including a plurality of phase gratings arranged in parallel lines extending along a circumference of said cylindrical surface to cause diffraction of an incident beam having a predetermined wavelength, wherein a beam incident on said diffractive grating pattern is divided into and emitted as a plurality of diffracted beams; wherein said plurality of phase gratings are of equal width and each of said plurality of phase gratings has a continuous nonlinear surface with a starting point and an ending point, such that a position of an ending point of one phase grating coincides with a position of a starting point of an adjacent phase grating, in a direction of arrangement of the gratings, said plurality of phase gratings causing phase differences in a wave front of said incident beam; wherein each of said plurality of phase gratings has an asymmetrical phase pattern, and wherein, a phase gap ΔP, representing a phase difference between an end point of each of said plurality of phase patterns and a beginning point of each of said plurality of phase patterns, in radians, is substantially equal for each of said plurality of phase gratings and satisfies: 0.7π<|ΔP|<1.2π.
 6. The diffractive optical element according to claim 5, wherein said plurality of phase grating are adjusted so that each of said divided diffracted beams have substantially the same intensity and no divided beam is emitted other than said diffracted beams having substantially the same intensity. 